The present invention relates to a die casting magnesium alloy having excellent heat resistance and castability.
For the purpose of weight saving, magnesium alloys have recently become of major interest in modes of transport, including automobiles.
As these magnesium alloys, particularly casting magnesium alloys, for example, Mgxe2x80x94Al alloys containing 2 to 6% by weight of Al (e.g. AM60B, AM50A, or AM20A defined in ASTM [American Society for Testing and Materials] standard) or Mgxe2x80x94Alxe2x80x94Zn alloys containing 8 to 10% by weight of Al and 1 to 3% by weight of Zn (e.g. AZ91D defined in ASTM standard) have been known. These magnesium alloys have good castability and can be applied to die casting.
However, in case such a magnesium alloy is used for parts for the proximity of an engine, the magnesium alloy is liable to cause yielding during use because of low creep strength at high temperature ranging from 125 to 175xc2x0 C., e.g. 150xc2x0 C., thus loosening bolts by which parts are clamped.
For example, typical die casting alloy AZ91D has poor creep strength, although it has good castability, tensile strength and corrosion resistance.
AE42 is known as a heat-resistant die casting alloy containing rare earth metals, but this alloy does not have good castability and also has poor creep strength.
Therefore, there have recently been suggested alloys wherein Ca is added to a Mgxe2x80x94Al alloy (Japanese Patent Application, First Publication No. Hei 7-11374 and Japanese Patent Application, First Publication No. Hei 9-291332).
However, these Mgxe2x80x94Alxe2x80x94Ca alloys have poor creep strength as compared with an aluminum alloy ADC12 (Al-1.5-3.5Cu-9.6-12.0 Si; corresponding to AA A384.0), although the creep strength is improved. Furthermore, these Mgxe2x80x94Alxe2x80x94Ca alloys have a problem that misrun and casting cracks are caused by deterioration of the die-castability. Although these alloys contain rare earth elements as essential components, the cost increases when rare earth elements are added in a large amount.
A thixocasting technique has recently been started to be applied to casting of magnesium alloys, unlike the die casting technique described above. This technique is considered to be effective to inhibit the occurrence of casting crack of the Mgxe2x80x94Alxe2x80x94Ca alloys because it is a method of performing injection molding in a semi-solid state.
However, this technique has never been completed and is not applied to automobile parts at present. Therefore, the die casting technique is still used exclusively as a method of casting Mg alloys.
As disclosed in Japanese Patent Application, First Publication No. Hei4-231435 (U.S. Pat. No. 5,147,603), the application relating to a magnesium alloy having a load at tensile rupture of at least 290 MPa and an elongation at tensile rupture of at least 5%, essentially consisting of 2 to 11% by weight of Al, 0 to 1% by weight of Mn, 0.1 to 6% by weight of Sr, the balance Mg, and less than 0.6% by weight of Si, less than 0.2% by weight of Cu, less than 0.1% by weight of Fe and less than 0.01% by weight of Ni as principal impurities has already been filed.
The magnesium alloys of this patent application are alloys having high mechanical strength and excellent corrosion resistance produced by a rapid solidification method, and is produced in the form of band, powder or tip from a molten alloy by a roller quenching, spraying or atomization method. The patent described above discloses a technique of obtaining a product having a desired shape by consolidating the resulting band, powder or tip to form a billet, and subjecting the billet to conventional extrusion or hydrostatic extrusion.
The alloy of the above patent application is an alloy produced by the rapid solidification process and has very high load at tensile rupture of 290 MPa or more, but this alloy is an alloy obtained only as a solid in the form of band, powder or tip by the rapid solidification process. In order to be formed into a desired shape of the product, alloy powders or alloy granules in the form of bands, powder or tips obtained by the rapid solidification process must be compacted by a heat consolidation molding method such as conventional extrusion or hydrostatic extrusion. Furthermore, finally obtainable shapes are limited.
An object to be attained by the present invention is to provide a die casting magnesium alloy which has excellent heat resistance and castability and also has excellent creep properties.
Another object to be attained by the present invention is to provide a die casting magnesium alloy which has the excellent properties described above and can be formed into a free shape by casting and can also be provided at low cost.
Still another object to be attained by the present invention is to provide a die casting magnesium alloy which is suited to the production of parts having a complicated shape around the engine or thin-wall parts and has excellent heat resistance and castability, and also has excellent creep properties.
As a result of an intensive study of the influence of additional elements on the castability and the creep strength of Mgxe2x80x94Alxe2x80x94Ca alloys containing Ca, the present inventors have found that the die-castability deteriorated by the addition of Ca can be remarkably improved and the creep strength can be further improved by adding Sr, thus completing the present invention.
The present invention has been attained based on such knowledge, and the objects described above can be attained by die casting magnesium alloys having excellent heat resistance and castability, comprising:
2 to 6% by weight (hereinafter xe2x80x9ctoxe2x80x9d indicates a numerical limitation range including an upper limit and a lower limit unless otherwise specified, and xe2x80x9c2 to 6% by weightxe2x80x9d represents the range of not less than 2% by weight and not more than 6% by weight) of Al, 0.3 to 2% by weight of Ca, 0.01 to 1% by weight of Sr, 0.1 to 1% by weight of Mn, the balance magnesium and unavoidable impurities.
The Al content was limited to xe2x80x9c2 to 6% by weightxe2x80x9d based on the results of the test described below.
When the Al content is not more than 6% by weight, a great portion of Al is incorporated into the matrix of Mg in the solid state. The tensile strength of the alloy is enhanced by solid-solution hardening. Also, the creep properties of the alloy are improved by the network-like structure of an Alxe2x80x94Ca compound crystallized out at grain boundary as a result of bonding with Ca. Al also improves the castability of the alloy.
However, when the Al content exceeds 6% by weight, the creep properties rapidly deteriorate. On the contrary, when the Al content is less than 2% by weight, the above effects (effect of improving the tensile strength of the alloy by solid-solution hardening, effect of improving the creep properties) are poor. Particularly, when the Al content is less than 2% by weight, the resulting alloy is liable to have low strength and poor practicability.
In light of the background described above, the Al content was set within a range from 2 to 6% by weight. The Al content is preferably within a range from 4.0 exclusive to 6% by weight, within the above range.
And the creep properties is improved with the increase of the Ca content. When the Ca content is less than 0.3% by weight, the improvement effect is small. However, when the Ca content exceeds 2% by weight, the casting crack is liable to occur.
In light of the background described above, the Ca content was set within a range from 0.3 to 2%by weight. The Ca content is preferably within a range from 0.5 to 1.5% by weight, within the above range.
Further the creep properties improved with the increase of the Sr content and it becomes hard to cause casting crack. This effect is small when the Sr content is less than 0.01% by weight. On the other hand, when the Sr content exceeds 1% by weight, the effect reaches the saturated state.
In the present invention, the Sr content was set within a range from 0.01 to 1% by weight. Under the circumstances described above, the Sr content is preferably within a range from 0.05 to 0.5% by weight, and more preferably within a range from 0.15 exclusive to 0.4% by weight, within the range described above.
In case Mn is added to this kind of an alloy, the corrosion resistance is improved and the creep strength is also improved. Furthermore, the proof stress, particularly high temperature proof stress is improved.
This effect is small when the Mn content is less than 0.1% by weight. However, when the Mn content exceeds 1% by weight, a large mount of a primary elemental Mn particle is crystallized. Therefore, the resulting alloy becomes brittle, thereby lowering the tensile strength.
For the reasons described above, the Mn content was set within a range from 0.1 to 1% by weight. The Mn content is more preferably within a range from 0.2 to 0.7% by weight.
The essential element in the Mg alloy of the present invention includes Al, Ca, Sr and Mn, in addition to Mg. The other elements are basically contained as unavoidable impurities.
However, when Si, Zn, and rare earth elements are contained in the proportion described below, the following advantages are also obtained.
Sometimes, the die casting magnesium alloy of the present invention further contains 0.1 to 1% by weight (preferably 0.2 to 0.6% by weight) of Si, in addition to the components described above. Sometimes, the die casting magnesium alloy further contains 0.2 to 1% by weight (preferably 0.4 to 0.8% by weight) of Zn, in addition to the components described above. Sometimes, the die casting magnesium alloy further contains 0.1 to 3% by weight (preferably 0.5 to 2.0% by weight, more preferably 0.8 to 1.5% by weight) of rare earth elements, in addition to the components described above.
Regarding the die casting magnesium alloy further containing Si in the proportion described above, it is made possible to obtain the advantage that the castability is further improved, thereby making it difficult to cause casting crack.
Regarding the die casting magnesium alloy further containing Zn in the proportion described above, it is made possible to obtain the advantage that the tensile strength is improved by solid-solution hardening.
Regarding the die casting magnesium alloy further containing rare earth elements in the proportion described above, it is made possible to obtain the advantage that the creep strength are further improved. Concretely, the alloy containing rare earth elements contains 2 to 6% by weight of Al, 0.3 to 2% by weight of Ca, 0.01 to 1% by weight of Sr, 0.1 to 1% by weight of Mn, 0.1 to 3% by weight of rare earth elements (one or more kinds of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), the balance Mg and unavoidable impurities. When the content of rare earth elements exceeds 3% by weight, casting crack increases and die-sticking becomes severe, thereby deteriorating the castability. Also, coarsening of the Alxe2x80x94RE compound in the constitution occurs, thereby deteriorating the mechanical properties. Furthermore, since rare earth elements are expensive elements, the smaller the amount, the better, in view of the cost.
The die casting magnesium alloy of the present invention such as Mgxe2x80x94Alxe2x80x94Ca-Mn-Sr alloy is produced by a general technique of melting the Mg alloy. For example, the alloy can be obtained by melting in an iron crucible using a protective gas such as SF6/CO2/Air.
The die casting magnesium alloy of the present invention has excellent mechanical properties such as tensile strength, proof stress, elongation, and the like and has excellent castability free from die-sticking during the casting, and also has excellent creep properties and corrosion resistance which are markedly excellent features for die casting magnesium alloys. According to the magnesium alloy of the present invention, it is made possible to obtain an excellent casting made of magnesium alloy, which is free from cracking and defects, even in case when thin-wall cast parts are produced.
The die casting magnesium alloy of the present invention is markedly preferred as an alloy to produce by die casting parts for the proximity of an engine, and can provide an excellent die casting product.